Memory utilizing oxide nanolaminates

摘要

Structures, systems and methods for transistors utilizing oxide nanolaminates are provided. One transistor embodiment includes a first source/drain region, a second source/drain region, and a channel region therebetween. A gate is separated from the channel region by a gate insulator. The gate insulator includes oxide insulator nanolaminate layers with charge trapping in potential wells formed by different electron affinities of the insulator nanolaminate layers.

权利要求

What is claimed is:

1. A programmable logic array, comprising:

a plurality of input lines for receiving an input signal;

a plurality of output lines; and

one or more arrays having a first logic plane and a second logic plane connected between the input lines and the output lines, wherein the first logic plane and the second logic plane comprise a plurality of logic cells arranged in rows and columns for providing a sum-of-products term on the output lines responsive to a received input signal, wherein each logic cell includes a transistor cell including:

a first source/drain region;

a second source/drain region;

a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator; and

wherein the gate insulator includes oxide insulator nanolaminate layers wherein at least one charge trapping layer in the oxide insulator nanolaminate layers is substantially amorphous.

2. The programmable logic array of claim 1, wherein the insulator nanolaminate layers include transition metal oxides.

3. The programmable logic array of claim 2, wherein the insulator nanolaminate layers including transition metal oxides formed by atomic layer deposition (ALD).

4. The programmable logic array of claim 1, wherein the insulator nanolaminate layers include silicon oxycarbide.

5. The programmable logic array of claim 4, wherein the insulator nanolaminate layers including silicon oxycarbide include silicon oxycarbide deposited using chemical vapor deposition.

6. A programmable logic array, comprising:

a plurality of input lines for receiving an input signal;

a plurality of output lines; and

one or more arrays having a first logic plane and a second logic plane connected between the input lines and the output lines, wherein the first logic plane and the second logic plane comprise a plurality of logic cells arranged in rows and columns for providing a sum-of-products term on the output lines responsive to a received input signal, wherein each logic cell includes a transistor cell including:

a first source/drain region;

a second source/drain region;

a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator; and

wherein the gate insulator includes oxide insulator nanolaminate layers and a transition metal oxide layer wherein the transition metal oxide layer is substantially amorphous.

7. The programmable logic array of claim 6, wherein the oxide insulator nanolaminate layers include silicon dioxide layers.

8. The programmable logic array of claim 7, wherein the transition metal oxide layer includes a hafnium oxide layer.

9. The programmable logic array of claim 7, wherein the transition metal oxide layer includes a zirconium oxide layer.

10. The programmable logic array of claim 7, wherein the transition metal oxide layer includes a titanium oxide layer.

11. The programmable logic array of claim 7, wherein the transition metal oxide layer includes a aluminum oxide layer.

12. A programmable logic array, comprising:

a plurality of input lines for receiving an input signal;

a plurality of output lines; and

one or more NOR arrays having a first logic plane and a second logic plane connected between the input lines and the output lines, wherein the first logic plane and the second logic plane comprise a plurality of logic cells arranged in rows and columns for providing a sum-of-products term on the output lines responsive to a received input signal, wherein each logic cell includes a transistor cell including:

a first source/drain region;

a second source/drain region;

a channel region between the first and the second source/drain regions, and a gate separated from the channel region by a gate insulator; and

wherein the gate insulator includes oxide insulator nanolaminate layers and a transition metal oxide layer and wherein approximately 100 electrons stored in the transition metal oxide layer produces a change in threshold voltage of the transistor cell of approximately 0.5 volts.

13. The programmable logic array of claim 12, wherein the insulator nanolaminate layers including transition metal oxides formed by atomic layer deposition (ALD).

14. The programmable logic array of claim 13, wherein the oxide insulator nanolaminate layers include silicon dioxide layers.

15. The programmable logic array of claim 14, wherein the transition metal oxide layer includes a hafnium oxide layer.

16. The programmable logic array of claim 14, wherein the transition metal oxide layer includes a zirconium oxide layer.

17. The programmable logic array of claim 14, wherein the transition metal oxide layer includes a titanium oxide layer.

18. The programmable logic array of claim 14, wherein the transition metal oxide layer includes a aluminum oxide layer.

说明书

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 11/458,854, filed Jul. 20, 2006,now U.S. Pat. No. 7,433,237, which is a Continuation of U.S. application Ser. No. 10/190,717, filed Jul. 8, 2002, now U.S. Pat. No. 7,221,586, which applications are incorporated herein by reference in its entirety.

This application is related to the following co-pending, commonly assigned U.S. patent applications: “Memory Utilizing Oxide-Nitride Nanolaminates,” , Ser. No. 10/190,689, and “Memory Utilizing Oxide-Conductor Nanolaminates,” Ser. No. 10/191,336, each of which disclosure is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor integrated circuits and, more particularly, to memory utilizing oxide nanolaminates.

BACKGROUND OF THE INVENTION

Many electronic products need various amounts of memory to store information, e.g. data. One common type of high speed, low cost memory includes dynamic random access memory (DRAM) comprised of individual DRAM cells arranged in arrays. DRAM cells include an access transistor, e.g a metal oxide semiconducting field effect transistor (MOSFET), coupled to a capacitor cell.

Another type of high speed, low cost memory includes floating gate memory cells. A conventional horizontal floating gate transistor structure includes a source region and a drain region separated by a channel region in a horizontal substrate. A floating gate is separated by a thin tunnel gate oxide. The structure is programmed by storing a charge on the floating gate. A control gate is separated from the floating gate by an intergate dielectric. A charge stored on the floating gate effects the conductivity of the cell when a read voltage potential is applied to the control gate. The state of cell can thus be determined by sensing a change in the device conductivity between the programmed and unprogrammed states.

With successive generations of DRAM chips, an emphasis continues to be placed on increasing array density and maximizing chip real estate while minimizing the cost of manufacture. It is further desirable to increase array density with little or no modification of the DRAM optimized process flow.

Multilayer insulators have been previously employed in memory devices. (See generally, U.S. Pat. No. 3,877,054, Boulin et al., Apr. 8, 1975, entitled “Semiconductor memory apparatus with a multi-layer insulator contacting the semiconductor,” and U.S. Pat. No. 3,964,085, Kahng et al., Jun. 15, 1976, entitled “Method for fabricating multilayer insulator-semiconductor memory apparatus”). The devices in the above references employed oxide-tungsten oxide-oxide layers. Other previously described structures described have employed charge-trapping layers implanted into graded layer insulator structures. (See generally, an article by DiMaria, D. J., “Graded or stepped energy band-gap-insulator MIS structures (GI-MIS or SI-MIS),” Journal of Applied Physics, 50(9), 5826-9 (September 1979); U.S. Pat. No. 4,217,601, DeKeersmaecker et al., Aug. 12, 1980, entitled “Non-volatile memory devices fabricated from graded or stepped energy band gap insulator MIM or MIS structure,” also RE31,083 DeKeersmaecker et al., Nov. 16, 1982, “Non-volatile memory devices fabricated from graded or stepped energy band gap insulator MIM or MIS structure;” and U.S. Pat. No. 5,768,192 Eitan, Jun. 16, 1998, entitled “Non-volatile semiconductor memory cell utilizing asymmetrical charge trapping”).

More recently oxide-nitride-oxide structures have been described for high density nonvolatile memories. (See generally, Etian, B. et al., “NROM: A novel localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Lett., 21(11), 543-545 (November 2000), and Eitan, B. et al., “Characterization of Channel Hot Electron Injection by the Subthreshold Slope of NROM device, IEEE Electron Device Lett., 22(11), 556-558 (November 2001)). All of these are variations on the original MNOS memory structure (see generally, Frohman-Bentchkowsky, D., “An integrated metal-nitride-oxide-silicon (MNOS) memory,” Proceedings of the IEEE, 57(6), 1190-2 (June 1969)) described by Fairchild Semiconductor in 1969 which was conceptually generalized to include trapping insulators in general for constructing memory arrays. (See generally, U.S. Pat. No. 3,665,423 Nakamuma et al., May 23, 1972, entitled “Memory matrix using MIS semiconductor element”).

Studies of charge trapping in MNOS structures have also been conducted by White and others. (See generally, White, M. H., “Direct tunneling in metal-nitride-oxide-silicon (MNOS) structures,” Conference: Program of the 31st physical electronics conference (abstracts), page: 1 pp., Publisher: U.S. Dept. Commerce, Washington, D.C., USA, 1971, viii+46 Pages, Sponsor: American Phys. Soc., division of electron and atomic phys, 15-17 Mar. 1971, Gaithersburg, Md., USA; White, M. H., Cricchi, J. R., “Characterization of thin-oxide MNOS memory transistors,” IEEE Transactions on Electron Devices, ED-19(12), 1280-8 (December 1972), Wei, L. S., Simmons, J. G. “Trapping, emission and generation in MNOS memory devices,” Solid-State Electronics, 17(6), 591-8 (June 1974), Ferris-Prabhu, A. V., “Charge transfer in layered insulators,” Solid-State Electronics, 16(9), 1086-7 (September 1973); Ferris-Prabhu, A. V., Lareau, L. J., “Amnesia in layered insulator FET memory devices,” Conference: 1973 International Electron Devices Meeting Technical Digest, Page: 75-7, Publisher: IEEE, New York, N.Y., USA, 1973, xvi+575 Pages, Sponsor: IEEE, 3-5 Dec. 1973, Washington, D.C., USA; Ferris-Prabhu, A. V., “Tunneling theories of non-volatile semiconductor memories,” Physica Status Solidi A, 35(1), 243-50 (16 May 1976)).

Some commercial and military applications utilized non-volatile MNOS memories. (See generally, Britton, J. et al., “Metal-nitride-oxide IC memory retains data for meter reader,” Electronics, 45(22); 119-23 (23 Oct. 1972); and Cricehi, J. R. et al., “Hardened MNOS/SOS electrically reprogrammable nonvolatile memory,” IEEE Transactions on Nuclear Science, ns-24(6), 2185-9 (December 1977), Conference: IEEE Annual Conference on Nuclear and Space Radiation Effects, Sponsor: IEEE, 12-15 Jul. 1977, Williamsburg, Va., USA).

However, these structures did not gain widespread acceptance and use due to their variability in characteristics and unpredictable charge trapping phenomena. They all depended upon the trapping of charge at interface states between the oxide and other insulator layers or poorly characterized charge trapping centers in the insulator layers themselves. Since the layers were deposited by CVD, they are thick, have poorly controlled thickness and large surface state charge-trapping center densities between the layers.

Thus, there is an ongoing need for improved DRAM technology compatible transistor cells. It is desirable that such transistor cells be fabricated on a DRAM chip with little or no modification of the DRAM process flow. It is further desirable that such transistor cells provide increased density and high access and read speeds.

SUMMARY OF THE INVENTION

The above mentioned problems for creating DRAM technology compatible transistor cells as well as other problems are addressed by the present invention and will be understood by reading and studying the following specification. This disclosure describes the use of oxide insulator nanolaminate layers with charge trapping in potential wells formed by the different electron affinities of the insulator layers. Two different types of materials are used for the nanolaminated insulator layers. The two different types of materials are transition metal oxides and silicon oxycarbide. In the case of transition metal oxide layers, these are formed by ALD and have atomic dimensions, or nanolaminates, with precisely controlled interfaces and layer thickness. In the case of silicon oxycarbide, these are deposited using chemical vapor deposition techniques since an ALD process has not yet been developed.

In particular, an embodiment of the present invention includes a transistor utilizing oxide nanolaminates. The transistor includes a first source/drain region, a second source/drain region, and a channel region therebetween. A gate is separated from the channel region by a gate insulator. The gate insulator includes oxide insulator nanolaminate layers with charge trapping in potential wells formed by different electron affinities of the insulator nanolaminate layers.

These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a metal oxide semiconductor field effect transistor (MOSFET) in a substrate according to the teachings of the prior art.

FIG. 1B illustrates the MOSFET of FIG. 1A operated in the forward direction showing some degree of device degradation due to electrons being trapped in the gate oxide near the drain region over gradual use.

FIG. 1C is a graph showing the square root of the current signal (Ids) taken at the drain region of the conventional MOSFET versus the voltage potential (VGS) established between the gate and the source region.

FIG. 2A is a diagram of an embodiment for a programmed MOSFET, having oxide insulator nanolaminate layers, which can be used as a transistor cell according to the teachings of the present invention.

FIG. 2B is a diagram suitable for explaining a method embodiment by which a MOSFET, having oxide insulator nanolaminate layers, can be programmed to achieve the embodiments of the present invention.

FIG. 2C is a graph plotting the current signal (Ids) detected at the drain region versus a voltage potential, or drain voltage, (VDS) set up between the drain region and the source region (Ids vs. VDS).

FIG. 3 illustrates a portion of an embodiment of a memory array according to the teachings of the present invention.

FIG. 4 illustrates an electrical equivalent circuit 400 for the portion of the memory array shown in FIG. 3.

FIG. 5 illustrates an energy band diagram for an embodiment of a gate stack according to the teachings of the present invention.

FIG. 6 is a graph which plots electron affinity versus the energy bandgap for various insulators.

FIGS. 7A-7B illustrates an embodiment for the operation of a transistor cell having oxide insulator nanolaminate layers according to the teachings of the present invention.

FIG. 8 illustrates the operation of a conventional DRAM cell.

FIG. 9 illustrates an embodiment of a memory device according to the teachings of the present invention.

FIG. 10 is a schematic diagram illustrating a conventional NOR-NOR programmable logic array.

FIG. 11 is a schematic diagram illustrating generally an architecture of one embodiment of a programmable logic array (PLA) with logic cells, having oxide insulator nanolaminate layers according to the teachings of the present invention.

FIG. 12 is a block diagram of an electrical system, or processor-based system, utilizing oxide nanolaminates constructed in accordance with the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.

The terms wafer and substrate used in the following description include any structure having an exposed surface with which to form the integrated circuit (IC) structure of the invention. The term substrate is understood to include semiconductor wafers. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures well known to one skilled in the art. The term conductor is understood to include semiconductors, and the term insulator is defined to include any material that is less electrically conductive than the materials referred to as conductors. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

FIG. 1A is useful in illustrating the conventional operation of a MOSFET such as can be used in a DRAM array. FIG. 1A illustrates the normal hot electron injection and degradation of devices operated in the forward direction. As is explained below, since the electrons are trapped near the drain they are not very effective in changing the device characteristics.

FIG. 1A is a block diagram of a metal oxide semiconductor field effect transistor (MOSFET) 101 in a substrate 100. The MOSFET 101 includes a source region 102, a drain region 104, a channel region 106 in the substrate 100 between the source region 102 and the drain region 104. A gate 108 is separated from the channel region 108 by a gate oxide 110. A sourceline 112 is coupled to the source region 102. A bitline 114 is coupled to the drain region 104. A wordline 116 is coupled to the gate 108.

In conventional operation, a drain to source voltage potential (Vds) is set up between the drain region 104 and the source region 102. A voltage potential is then applied to the gate 108 via a wordline 116. Once the voltage potential applied to the gate 108 surpasses the characteristic voltage threshold (Vt) of the MOSFET a channel 106 forms in the substrate 100 between the drain region 104 and the source region 102. Formation of the channel 106 permits conduction between the drain region 104 and the source region 102, and a current signal (Ids) can be detected at the drain region 104.

In operation of the conventional MOSFET of FIG. 1A, some degree of device degradation does gradually occur for MOSFETs operated in the forward direction by electrons 117 becoming trapped in the gate oxide 110 near the drain region 104. This effect is illustrated in FIG. 1B. However, since the electrons 117 are trapped near the drain region 104 they are not very effective in changing the MOSFET characteristics.

FIG. 1C illustrates this point. FIG. 1C is a graph showing the square root of the current signal (Ids) taken at the drain region versus the voltage potential (VGS) established between the gate 108 and the source region 102. The change in the slope of the plot of √{square root over (Ids)} versus VGS represents the change in the charge carrier mobility in the channel 106.

In FIG. 1C, ΔVT represents the minimal change in the MOSFET's threshold voltage resulting from electrons gradually being trapped in the gate oxide 110 near the drain region 104, under normal operation, due to device degradation. This results in a fixed trapped charge in the gate oxide 110 near the drain region 104. Slope 1 represents the charge carrier mobility in the channel 106 for FIG. 1A having no electrons trapped in the gate oxide 110. Slope 2 represents the charge mobility in the channel 106 for the conventional MOSFET of FIG. 1B having electrons 117 trapped in the gate oxide 110 near the drain region 104. As shown by a comparison of slope 1 and slope 2 in FIG. 1C, the electrons 117 trapped in the gate oxide 110 near the drain region 104 of the conventional MOSFET do not significantly change the charge mobility in the channel 106.

There are two components to the effects of stress and hot electron injection. One component includes a threshold voltage shift due to the trapped electrons and a second component includes mobility degradation due to additional scattering of carrier electrons caused by this trapped charge and additional surface states. When a conventional MOSFET degrades, or is “stressed,” over operation in the forward direction, electrons do gradually get injected and become trapped in the gate oxide near the drain. In this portion of the conventional MOSFET there is virtually no channel underneath the gate oxide. Thus the trapped charge modulates the threshold voltage and charge mobility only slightly.

The inventors have previously described programmable memory devices and functions based on the reverse stressing of MOSFET's in a conventional CMOS process and technology in order to form programmable address decode and correction. (See generally, L. Forbes, W. P. Noble and E. H. Cloud, “MOSFET technology for programmable address decode and correction,” application Ser. No. 09/383,804). That disclosure, however, did not describe write once read only memory solutions, but rather address decode and correction issues. The inventors also describe write once read only memory cells employing charge trapping in gate insulators for conventional MOSFETs and write once read only memory employing floating gates. The same are described in co-pending, commonly assigned U.S. patent applications, entitled “Write Once Read Only Memory Employing Charge Trapping in Insulators,” Ser. No. 10/177,077, and “Write Once Read Only Memory Employing Floating Gates,” Ser. No. 10/177,083. The present application, however, describes transistor cells having oxide insulator nanolaminate layers and used in integrated circuit device structures.

According to the teachings of the present invention, normal flash memory type cells can be programmed by operation in the reverse direction and utilizing avalanche hot electron injection to trap electrons in the gate insulator nanolaminate. When the programmed floating gate transistor is subsequently operated in the forward direction the electrons trapped in the gate insulator nanolaminate cause the channel to have a different threshold voltage. The novel programmed flash memory type transistors of the present invention conduct significantly less current than conventional flash cells which have not been programmed. These electrons will remain trapped in the gate insulator nanolaminate unless negative control gate voltages are applied. The electrons will not be removed from the gate insulator nanolaminate when positive or zero control gate voltages are applied. Erasure can be accomplished by applying negative control gate voltages and/or increasing the temperature with negative control gate bias applied to cause the trapped electrons in the gate insulator nanolaminate to be re-emitted back into the silicon channel of the MOSFET.

FIG. 2A is a diagram of an embodiment for a programmed transistor cell 201 having oxide insulator nanolaminate layers according to the teachings of the present invention. As shown in FIG. 2A the transistor cell 201 includes a transistor in a substrate 200 which has a first source/drain region 202, a second source/drain region 204, and a channel region 206 between the first and second source/drain regions, 202 and 204. In one embodiment, the first source/drain region 202 includes a source region 202 for the transistor cell 201 and the second source/drain region 204 includes a drain region 204 for the transistor cell 201. FIG. 2A further illustrates the transistor cell 201 having oxide insulator nanolaminate layers 208 separated from the channel region 206 by an oxide 210. An sourceline or array plate 212 is coupled to the first source/drain region 202 and a transmission line 214 is coupled to the second source/drain region 204. In one embodiment, the transmission line 214 includes a bit line 214. Further as shown in FIG. 2A, a gate 216 is separated from the oxide insulator nanolaminate layers 208 by another oxide 218.

As stated above, transistor cell 201 illustrates an embodiment of a programmed transistor. This programmed transistor has a charge 217 trapped in potential wells in the oxide insulator nanolaminate layers 208 formed by the different electron affinities of the insulators 208, 210 and 218. In one embodiment, the charge 217 trapped on the floating gate 208 includes a trapped electron charge 217.

FIG. 2B is a diagram suitable for explaining the method by which the oxide insulator nanolaminate layers 208 of the transistor cell 201 of the present invention can be programmed to achieve the embodiments of the present invention. As shown in FIG. 2B the method includes programming the floating gate transistor. Programming the floating gate transistor includes applying a first voltage potential V1 to a drain region 204 of the floating gate transistor and a second voltage potential V2 to the source region 202.

In one embodiment, applying a first voltage potential V1 to the drain region 204 of the floating gate transistor includes grounding the drain region 204 of the floating gate transistor as shown in FIG. 2B. In this embodiment, applying a second voltage potential V2 to the source region 202 includes biasing the array plate 212 to a voltage higher than VDD, as shown in FIG. 2B. A gate potential VGS is applied to the control gate 216 of the transistor. In one embodiment, the gate potential VGS includes a voltage potential which is less than the second voltage potential V2, but which is sufficient to establish conduction in the channel 206 of the transistor between the drain region 204 and the source region 202. As shown in FIG. 2B, applying the first, second and gate potentials (V1, V2, and VGS respectively) to the transistor creates a hot electron injection into the oxide insulator nanolaminate layers 208 of the transistor adjacent to the source region 202. In other words, applying the first, second and gate potentials (V1, V2, and VGS respectively) provides enough energy to the charge carriers, e.g. electrons, being conducted across the channel 206 that, once the charge carriers are near the source region 202, a number of the charge carriers get excited into the oxide insulator nanolaminate layers 208 adjacent to the source region 202. Here the charge carriers become trapped in potential wells in the oxide insulator nanolaminate layers 208 formed by the different electron affinities of the insulators 208, 210 and 218.

In an alternative embodiment, applying a first voltage potential V1 to the drain region 204 of the transistor includes biasing the drain region 204 of the transistor to a voltage higher than VDD. In this embodiment, applying a second voltage potential V2 to the source region 202 includes grounding the sourceline or array plate 212. A gate potential VGS is applied to the control gate 216 of the transistor. In one embodiment, the gate potential VGS includes a voltage potential which is less than the first voltage potential V1, but which is sufficient to establish conduction in the channel 206 of the transistor between the drain region 204 and the source region 202. Applying the first, second and gate potentials (V1, V2, and VGS respectively) to the transistor creates a hot electron injection into the oxide insulator nanolaminate layers 208 of the transistor adjacent to the drain region 204. In other words, applying the first, second and gate potentials (V1, V2, and VGS respectively) provides enough energy to the charge carriers, e.g. electrons, being conducted across the channel 206 that, once the charge carriers are near the drain region 204, a number of the charge carriers get excited into the oxide insulator nanolaminate layers 208 adjacent to the drain region 204. Here the charge carriers become trapped in potential wells in the oxide insulator nanolaminate layers 208 formed by the different electron affinities of the insulators 208, 210 and 218, as shown in FIG. 2A.

In one embodiment of the present invention, the method is continued by subsequently operating the transistor in the forward direction in its programmed state during a read operation. Accordingly, the read operation includes grounding the source region 202 and precharging the drain region a fractional voltage of VDD. If the device is addressed by a wordline coupled to the gate, then its conductivity will be determined by the presence or absence of stored charge in the oxide insulator nanolaminate layers 208. That is, a gate potential can be applied to the gate 216 by a wordline 220 in an effort to form a conduction channel between the source and the drain regions as done with addressing and reading conventional DRAM cells.

However, now in its programmed state, the conduction channel 206 of the transistor will have a higher voltage threshold and will not conduct.

FIG. 2C is a graph plotting a current signal (IDS) detected at the second source/drain region 204 versus a voltage potential, or drain voltage, (VDS) set up between the second source/drain region 204 and the first source/drain region 202 (IDS vs. VDS). In one embodiment, VDS represents the voltage potential set up between the drain region 204 and the source region 202. In FIG. 2C, the curve plotted as D1 represents the conduction behavior of a conventional transistor which is not programmed according to the teachings of the present invention. The curve D2 represents the conduction behavior of the programmed transistor, described above in connection with FIG. 2A, according to the teachings of the present invention. As shown in FIG. 2C, for a particular drain voltage, VDS, the current signal (IDS2) detected at the second source/drain region 204 for the programmed transistor (curve D2) is significantly lower than the current signal (IDS1) detected at the second source/drain region 204 for the conventional transistor cell which is not programmed according to the teachings of the present invention. Again, this is attributed to the fact that the channel 206 in the programmed transistor of the present invention has a different voltage threshold.

Some of these effects have recently been described for use in a different device structure, called an NROM, for flash memories. This latter work in Israel and Germany is based on employing charge trapping in a silicon nitride layer in a non-conventional flash memory device structure. (See generally, B. Eitan et al., “Characterization of Channel Hot Electron Injection by the Subthreshold Slope of NROM device,” IEEE Electron Device Lett., Vol. 22, No. 11, pp. 556-558, (November 2001); B. Etian et al., “NROM: A novel localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Lett., Vol. 21, No. 11, pp. 543-545, (November 2000)). Charge trapping in silicon nitride gate insulators was the basic mechanism used in MNOS memory devices (see generally, S. Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506), charge trapping in aluminum oxide gates was the mechanism used in MIOS memory devices (see generally, S. Sze, Physics of Semiconductor Devices, Wiley, N.Y., 1981, pp. 504-506), and the present inventors have previously disclosed charge trapping at isolated point defects in gate insulators (see generally, L. Forbes and J. Geusic, “Memory using insulator traps,” U.S. Pat. No. 6,140,181, issued Oct. 31, 2000). However, none of the above described references addressed forming transistor cells utilizing charge trapping in potential wells in oxide insulator nanolaminate layers formed by the different electron affinities of the insulators.

FIG. 3 illustrates an embodiment for a portion of a memory array 300 according to the teachings of the present invention. The memory in FIG. 3, is shown illustrating a number of vertical pillars, or transistor cells, 301-1, 301-2, . . . , 301-N, formed according to the teachings of the present invention. As one of ordinary skill in the art will appreciate upon reading this disclosure, the number of vertical pillar are formed in rows and columns extending outwardly from a substrate 303. As shown in FIG. 3, the number of vertical pillars, 301-1, 301-2, . . . , 301-N, are separated by a number of trenches 340. According to the teachings of the present invention, the number of vertical pillars, 301-1, 301-2, . . . , 301-N, serve as transistors including a first source/drain region, e.g. 302-1 and 302-2 respectively. The first source/drain region, 302-1 and 302-2, is coupled to a sourceline 304. As shown in FIG. 3, the sourceline 304 is formed in a bottom of the trenches 340 between rows of the vertical pillars, 301-1, 301-2, . . . , 301-N. According to the teachings of the present invention, the sourceline 304 is formed from a doped region implanted in the bottom of the trenches 340. A second source/drain region, e.g. 306-1 and 306-2 respectively, is coupled to a bitline (not shown). A channel region 305 is located between the first and the second source/drain regions.

As shown in FIG. 3, oxide insulator nanolaminate layers, shown generally as 309, are separated from the channel region 305 by a first oxide layer 307 in the trenches 340 along rows of the vertical pillars, 301-1, 301-2, . . . , 301-N. In the embodiment shown in FIG. 3, a wordline 313 is formed across the number of pillars and in the trenches 340 between the oxide insulator nanolaminate layers 309. The wordline 313 is separated from the pillars and the oxide insulator nanolaminate layers 309 by a second oxide layer 317.

FIG. 4 illustrates an electrical equivalent circuit 400 for the portion of the memory array shown in FIG. 3. As shown in FIG. 4, a number of vertical transistor cells, 401-1, 401-2, . . . , 401-N, are provided. Each vertical transistor cell, 401-1, 401-2, . . . , 401-N, includes a first source/drain region, e.g. 402-1 and 402-2, a second source/drain region, e.g. 406-1 and 406-2, a channel region 405 between the first and the second source/drain regions, and oxide insulator nanolaminate layers, shown generally as 409, separated from the channel region by a first oxide layer.

FIG. 4 further illustrates a number of bit lines, e.g. 411-1 and 411-2. According to the teachings of the present invention as shown in the embodiment of FIG. 4, a single bit line, e.g. 411-1 is coupled to the second source/drain regions, e.g. 406-1 and 406-2, for a pair of transistor cells 401-1 and 401-2 since, as shown in FIG. 3, each pillar contains two transistor cells. As shown in FIG. 4, the number of bit lines, 411-1 and 411-2, are coupled to the second source/drain regions, e.g. 406-1 and 406-2, along rows of the memory array. A number of word lines, such as wordline 413 in FIG. 4, are coupled to a gate 412 of each transistor cell along columns of the memory array. According to the teachings of the present invention, a number of sourcelines, 415-1, 415-2, . . . , 415-N, are formed in a bottom of the trenches between rows of the vertical pillars, described in connection with FIG. 3, such that first source/drain regions, e.g. 402-2 and 402-3, in column adjacent transistor cells, e.g. 401-2 and 401-3, separated by a trench, share a common sourceline, e.g. 415-1. And additionally, the number of sourcelines, 415-1, 415-2, . . . , 415-N, are shared by column adjacent transistor cells, e.g. 401-2 and 401-3, separated by a trench, along rows of the memory array 400. In this manner, by way of example and not by way of limitation referring to column adjacent transistor cells, e.g. 401-2 and 401-3, separated by a trench, when one column adjacent transistor cell, e.g. 401-2, is being read its complement column adjacent transistor cell, e.g. 401-3, can operate as a reference cell.

Method of Formation

This disclosure describes the use of oxide insulator nanolaminate layers with charge trapping in potential wells formed by the different electron affinities of the insulator layers. Two different types of materials are used for the nanolaminated insulator layers, transition metal oxides and silicon oxycarbide. (See generally, Wilk, G. D. et al., “High-k gate dielectric: Current status and materials properties considerations,” Jour. Appl. Phys., 89(10), 5243-75 (2001); Robertson, J., “Band offsets of wide-band-gap oxides and implications for future electronic devices,” J. Vac. Sci. Technol. B, 18(3), 1785-91 (2000); Luan, H. F. et al., “High quality Ta₂O₅ gate dielectrics with T_ox, equil. 10 Angstroms,” IEDM Tech. Digest, 141-144 (1999); Zhu, W. J. et al., “Current transport in metal/hafnium oxide/silicon structure,” IEEE Electron Device Letters, 23(2), 97-99 (2002) for discussion on transition metal properties). (See generally, Yoder, M. N., “Wide bandgap semiconductor materials and devices,” IEEE Trans. on Electron Devices, 43, 1633-36 (October 1996); Ahn, K. Y. and Forbes, L., “Porous silicon oxycarbide integrated circuit insulator,” U.S. Pat. No. 6,313,518; Forbes, L. et al., “Transistor with silicon oxycarbide gate and methods of fabrication and use,” U.S. Pat. No. 5,886,368, for discussion on silicon oxycarbide properties).

In the case of transition metal oxide layers, in embodiments of the present invention these are formed by ALD and have atomic dimensions, or nanolaminates, with precisely controlled interfaces and layer thickness. In the case of silicon oxycarbide, in embodiments of the present invention these are deposited using chemical vapor deposition techniques since an ALD process has not yet been developed.

Transition Metal Oxides

Atomic Layer Deposition

Embodiments of the present invention use the atomic controlled deposition method to form the gate insulators if transition metal oxides are employed for the electron trapping layer. Atomic Layer Deposition (ALD), developed in the early 70s, is a modification of CVD and can also be called as “alternately pulsed-CVD.” (See generally, Ofer Sneh et al., “Thin film atomic layer deposition equipment for semiconductor processing,” Thin Solid Films, 402, 248-261 (2002)). Gaseous precursors are introduced one at a time to the substrate surface, and between the pulses the reactor is purged with an inert gas or evacuated. In the first reaction step, the precursor is saturatively chemisorbed at the substrate surface, and during the subsequent purging the precursor is removed from the reactor. In the second step, another precursor is introduced on the substrate and the desired films growth reaction takes place. After that the reaction byproducts and the precursor excess are purged out from the reactor. When the precursor chemistry is favorable, i.e., the precursor adsorb and react with each other aggressively, one ALD cycle can be preformed in less than one second in the properly designed flow type reactors.

The striking feature of ALD is the saturation of all the reaction and purging steps which makes the growth self-limiting. This brings the large area uniformity and conformality, the most important properties of ALD, as shown in very different cases, viz. planar substrates, deep trenches, and in the extreme cases of porous silicon and high surface area silica and alumina powers. Also the control of the film thickness is straightforward and can be made by simply calculating the growth cycles. ALD was originally developed to manufacture luminescent and dielectric films needed in electroluminescent displays, and a lot of effort has been put to the growth of doped zinc sulfide and alkaline earth metal sulfide films. Later ALD has been studied for the growth of different epitaxial II-V and II-VI films, nonepitaxial crystalline or amorphous oxide and nitride films are their multilayer structures. There has been considerable interest towards the ALD growth of silicon and germanium films but due to the difficult precursor chemistry, the results have not been very successful.

Reaction sequence ALD (RS-ALD) films have several unique and unmatched advantages:

• • Continuity at the interface avoiding poorly defined nucleating regions that are typical for CVD (<20 Å) and PVD (<50 Å) films. To achieve this continuity, the substrate surface must be activated to react directly with the first exposure of RS-ALD precursor.
  • Unmatched conformality over toughest substrate topologies with robust processes that can only be achieved with a layer-by-layer deposition technique.
  • Typically, low temperature and mildly oxidizing processes. This is thought to be a major advantage for gate insulator processing where deposition of non-silicon based dielectrics without oxidizing the substrate (with the oxidation-precursor) is a major concern.
  • RS-ALD ability to engineer multilayer laminate films, possibly down to monolayer resolution, as well as alloy composite films appear to be unique. This ability comes from the combination of being able to control deposition with monolayer precision and the ability to deposit continuous monolayers of amorphous films (that is unique to RS-ALD).
  • Unprecedented process robustness. RS-ALD processes are free of first wafer effects and the chamber dependence. Accordingly, RS-ALD processes will be easier to transfer from development to production and from 200 to 300 mm wafer size.
  • Thickness depends solely on the number of cycles. Thickness can be “dialed in” as a simple recipe change bearing no need for additional process development upon technology generation advance.

    
    
    

    (See generally, Shunsuke Morishita et al., “Atomic-Layer Chemical-Vapor-Deposition of SiO₂ by Cyclic Exposure of CHOSi(NCO)₃ and H₂O₂,” Jpn. J. Appl. Phys., 34, 5738-42 (1955)).

    
    
    

    ALD Processes for Disclosed Structure with Transition Metal Oxides

FIG. 5 illustrates an energy band diagram for an embodiment of a gate stack according to the teachings of the present invention. As shown in FIG. 5, the embodiment consists of insulator stacks, 501-1, 501-2 and 501-3, e.g. SiO₂/oxide insulator nanolaminate layers/SiO₂. The first and the last layer, 501-1 and 501-3, are done by atomic layer deposition. In some embodiments, layers 501-1 and 501-3 are deposited by cyclic exposures of CH₃OSi(NCO)₃ and H₂O₂ at room temperature. (See generally, Shunsuke Morishita et al., “Atomic-Layer Chemical-Vapor-Deposition of SiO₂ by Cyclic Exposure of CHOSi(NCO)₃ and H₂O₂,” Jpn. J. Appl. Phys., 34, 5738-42 (1955)). In this embodiment, the deposition rate is saturated at about 2 Å/cycle, i.e., equal to the ideal quasi-monolayer/cycle. In one example the surface roughness for 100 deposition cycles is found to be less than ±10 Å by atomic force microscopy.

In the Morishita reference, metallic oxides were used. Those metallic oxides included HfO₂, Zr₂O₂, Ta₂O₃, La- and Y-based oxides, TiO₂, and Al₂O₃ and are discussed in the following paragraphs.

Recently a special technical meeting on ‘Atomic Layer Deposition’ was held by the American Vacuum Society. (See generally, Forsgren, Katarina et al., “Atomic Layer Deposition of HfO₂ using hafnium iodide,” one page summary of work, Conference held in Monterey, Calif., May 14-15, 2001). In the printed form, the above reference showed a summary of HfO₂ growth using HfI₄ for the first time, which results in a high melting material with a low leakage current and dielectric constant of 16-30. Together with a high chemical stability in contact with silicon, this makes HfO₂ a possible replacement for SiO₂ as a gate oxide. Previous work in the Forsgren group has shown that iodides can be used as metal sources in ALD of high-purity oxide films, e.g., Ta₂O₅, TaO₂, ZrO₂. Their study demonstrates the use of HfI₄ in ALD for the first time. In a recent paper by Zhang et al., they published work on thin stacks comprised of alternate layers of Ta₂O₅/HfO₂, Ta₂O₅/ZrO₂, and ZrO₂/HfO₂. (See generally, Zhang, H. and Solanki, R., “Atomic Layer Deposition of High Dielectric Constant Nanolaminates,” Jour. of the Electrochemical Soc., 148(4) F63-F66 (2001)). Zhang et al. reported the thin stacks as high-permittivity insulators for possible gate applications. These thin layers were deposited on silicon substrates using atomic layer deposition. Nanolaminate with silicon oxide equivalent thickness of about 2 nm had dielectric constants of around ten and leakage current densities at 1 MV/cm of around 10⁻⁸ Å/cm². Of the three kinds of nanolaminates investigated, ZrO₂/HfO₂ structure showed the highest breakdown field and the lowest leakage current. Zhang et al. report that by growing nanolaminates of high-permittivity thin films, leakage current of about 5×10⁻⁷ Å/cm² and k values of around 10 can be obtained for films of equivalent SiO₂ thickness, e.g. less than 3 nm.

In embodiments of the present invention, nanolaminates of HfO₂ and ZrO₂ are described as a dielectric material in new device structures with silicon oxide-metal oxide-silicon oxide insulator nanolaminates. Films with ALD of HfO₂ are prepared with substrate temperature of 225-500° C. using HfI₄ as precursor, instead of HfCl₄. (See generally, Forsgren, Katarina et al., “Atomic Layer Deposition of HfO₂, using hafnium iodide,” one page summary of work, Conference held in Monterey, Calif., May 14-15, 2001). Another process temperature for the HfO₂ is at 325° C. as practiced by Kukli et al. (See generally, Kukli, Kaupo et al., “Dielectric Properties of Zirconium Oxide Grown by Atomic Layer Deposition from Iodide Precursor,” Jour. of the Electrochemical Soc., 148(12), F227-F232 (2001)). For deposition of ALD ZrO₂, an alternative precursor of ZrI₄ would be used instead of ZrCl₄. ZrO₂ films were previously grown from ZrI₄ and H₂O—H₂O using the same atomic layer deposition technique. (See generally, Carter, R. J. et al., “Electrical Characterization of High-k Materials Prepared by Atomic Layer CVD,” IWGI, 94-99 (2001)). The breakdown field exceeded 2 MV/cm in the films grown at 325-500° C. The relative permittivity measured at 10 kHz was 20-24 in the films deposited at 275-325° C. The dissipation factor of these films was as low as 0.02-0.03. Thus, for the deposition of nanolaminates, a temperature of 250 to 325° C. would be recommended. Other references for ZrO₂ may be useful to note. (See generally, Kukli, Kaupo et al., “Tailoring the dielectric properties of HfO₂—Ta₂O₃ nanolaminates,” Appl. Phys. Lett., 68(26), 3737-39 (1996)).

Guha et al. reported on the electrical and microstructural characteristics of La- and Y-based oxides grown on silicon substrates by ultrahigh vacuum atomic beam deposition. (See generally, Guha, S. et al., “Atomic beam deposition of lanthanum- and yttrium-based oxide thin films for gate dielectrics,” Appl. Phys. Lett., 77(17), 2710-2712 (2000)). The Guha et al. group was interested in examining the potential of lanthanum- and yttrium-based oxide-thin films as alternate gate dielectrics for Si complementary metal oxide semiconductor technology. Guha et al. examined the issue of the polycrystallinity and interfacial silicon oxide formation in these films and their effect on the leakage currents and the ability to deposit films with low electrical thinness. They found that the interfacial SiO₂ is much thicker at ˜1.5 nm for the Y-based oxide compared to the La-based oxide where the thickness <0.5 nm. They also showed that while the Y-based oxide films show excellent electrical properties, the La-based films exhibit a large flat band voltage shift indicative of positive charge in the films. In embodiments of the present invention, nanolaminates of HfO₂ and ZrO₂ are also described as a dielectric material in new device structures with silicon oxide-metal oxide-silicon oxide insulator nanolaminates.

Niilisk et al. studied the initial growth of TiO₂ films by ALD. (See generally, Niilisk, A. et al., “Atomic-scale optical monitoring of the initial growth of TiO₂ thin films,” Proc. of the SPIE, 4318, 72-77 (2001)). The initial atomic-layer-chemical-vapor-deposition growth of titanium dioxide from TiCl₄ and water on quartz glass substrate was monitored in real time by incremental dielectric reflection. In the Niilisk et al. reference an interesting means for beginning the growth from the very beginning into a time-homogeneous mode was proposed and preliminarily studied. The means for beginning the growth from the very beginning into a time-homogeneous mode consists of an in situ TiCl₄-treatment procedure. The crystal structure and surface morphology of the prepared ultrathin films were characterized by Niilisk et al. In embodiments of the present invention, nanolaminates of TiO₂ are also described as a dielectric material in new device structures with silicon oxide-metal oxide-silicon oxide insulator nanolaminates.

Further, in embodiments of the present invention, nanolaminates of Al₂O₃ are described as a dielectric material for new device structures with silicon oxide-metal oxide-silicon oxide insulator nanolaminates. In these embodiments, Al₂O₃ can be deposited by ALD. (See generally, DiMaria, D. J. “Graded or stepped energy band-gap-insulator MIS structures (GI-MIS or SI-MIS),” Journal of Applied Physics, 50(9), 5826-9 (September 1979) for one technique appropriate for performing the ALD deposition).

Silicon Oxycarbide

Silicon oxycarbide is a wide band gap semiconductor, with a band gap energy which can vary between that of silicon carbide and that of silicon oxide. (See generally, Yoder, M. N., Wide bandgap semiconductor materials and devices,” IEEE Trans. on Electron Devices, 43, 1633-1636 (October 1996)). FIG. 6 is a graph which plots electron affinity versus the energy bandgap for various insulators. That is FIG. 6 illustrates the inventors estimates of the variation of the electron affinity with the bandgap energy. If the insulator is crystalline and has a small band gap, near that of silicon carbide, then the insulator can be doped and be conductive, however if undoped and in an amorphous state with a larger band gap, it will be an insulator. The inventors of the present case, Ahn, K. Y. and Forbes, L., have previously described silicon oxycarbide for use as an insulator in integrated circuits. (See generally, U.S. Pat. No. 6,313,518, by Ahn, K. Y. and Forbes, L., entitled “Porous silicon oxycarbide integrated circuit insulator”). The inventors of the present case, Ahn, K. Y. and Forbes, L., have previously described doped and microcrystalline silicon oxycarbide to be conductive for use as a gate material. (See generally, U.S. Pat. No. 5,886,368, by Forbes, L. et al., entitled “Transistor with silicon oxycarbide gate and methods of fabrication and use”). Additionally, silicon oxycarbide has been described for passivation of integrated circuit chips. (See generally, U.S. Pat. No. 5,530,581, by S. F. Cogan, entitled “Protective overlay material and electro-optical coating using same”).

Silicon oxycarbide can be deposited by chemical vapor deposition, CVD, techniques. (See generally, Fauchet, P. M. et al., “Optoelectronics and photovoltaic applications of microcrystalline SiC,” Symp. on Materials Issues in Microcrystalline Semiconductors, pp. 291-2 (1989); Demichelis, F. et al., “Physical properties of undoped and doped microcrystalline SiC:H deposited by PECVD,” Symp. on Amorphous Silicon Technology, pp. 413-18 (1991); Demichelis, F. et al., “Influence of doping on the structural and optoelectronic properties of amorphous and microcrystalline silicon carbine,” J. Appl. Phys., 72(4), 1327-33 (1992); Chang, C. Y. et al., “Novel passivation dielectrics—the boron- or phosphorus-doped hydrogenated amorphous silicon carbide films,” J. Electrochemical Society, 132(2), 418-22 (February 1995); Martins, R. et al., “Transport properties of doped silicon Oxycarbide microcrystalline films produced by spatial separation techniques,” Solar Energy Materials and Solar Cells, 41-42, 493-517 (June, 1996); Martins, R. et al., “Wide band-gap microcrystalline silicon thin films,” Diffusion Defect Data Part B (Solid State Phenomena), Vol. 44-46, pt. 2, p. 299-346 (1995); Renlund, G. M. et al., “Silicon oxycarbide glasses, I. Preparation and chemistry, J. Materials Research, 6(12), 2716-22 (December 1991); Renlund, G. M. et al., “Silicon oxycarbide glasses, II. Structure and properties,” J. Materials Research, 6(12), 2723-34 (December 1991)). In the silicon oxycarbide embodiments of the present invention, an initial gate oxide is grown by thermal oxidation of silicon and then the silicon oxycarbide and final oxide layer is deposited by CVD.

Memory Devices

According to the teachings of the present invention, the gate insulator structure shown in FIG. 5 is employed in a wide variety of different flash memory type devices. That is, in embodiments of the present invention, the gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide, is used in place of the gate structure provided in the following commonly assigned patents: U.S. Pat. Nos. 5,936,274; 6,143,636; 5,973,356; 6,238,976; 5,991,225; 6,153,468; and 6,124,729.

In embodiments of the present invention, the gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide, is used in place of the gate structure provided in the following commonly assigned pending applications: Forbes, L., “Write once read only memory based on DRAM technology employing charge trapping in gate insulators,” application Ser. No. 10/177,077; Forbes, L., “Write once read only memory based on a modification of DRAM technology employing floating gates,” application Ser. No. 10/177,083; Forbes, L., “Write once read only memory with long retention for archival storage,” application Ser. No. 10/177,213; Forbes, L., “Nanoncrystal write once read only memory with long retention for archival storage,” application Ser. No. 10/177,214; Forbes, L., “Ferroelectric write once read only memory with long retention for archival storage,” application Ser. No. 10/177,082; Forbes, L., “Vertical NROM having a storage density of 1 bit/1F² flash memory cell,” application Ser. No. 10/177,208; Forbes, L., “Multistate NROM having a storage density much greater than 1 bit/1F²,” application Ser. No. 10/177,211; Forbes, L., “NOR flash memory cell with high storage density,” application Ser. No. 10/177,483.

According to the teachings of the present invention, embodiments of the novel transistor herein, which are substituted for the gate structures described in the references above, are programmed by grounding a source line and applying a gate voltage and a voltage to the drain to cause channel hot electron injection. To read the memory state, the drain and ground or source have the normal connections and the conductivity of the transistor determined using low voltages so as not to disturb the memory state. The devices can be erased by applying a large negative voltage to the gate.

In embodiments of the present invention, the gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide, is used in place of the gate structure provided in the following commonly assigned patents: U.S. Pat. Nos. 5,936,274, 6,143,636, 5,973,356 and 6,238,976 (vertical flash memory devices with high density); U.S. Pat. Nos. 5,991,225 and 6,153,468 (programmable memory address and decode circuits); and U.S. Pat. No. 6,124,729 (programmable logic arrays).

Further, in embodiments of the present invention, the gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide, is used in place of the gate structure provided in the following U.S. patents: Etian, B. et al., “NROM: A novel localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Lett., 21(11), 543-545 (November 2000); Eitan, B. et al., “Characterization of Channel Hot Electron Injection by the Subthreshold Slope of NROM device, IEEE Electron Device Lett., 22(11), 556-558 (November 2001); Maayan, E. et al., “A 512 Mb NROM Flash Data Storage Memory with 8 MB/s Data Rate,” Dig. IEEE Int. Solid-State Circuits Conf., 100-101 (2002). In these embodiments, the gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide used in place of the gate structures in those references, can be programmed in the reverse direction and read in the forward direction to obtain more sensitivity in the device characteristics to the stored charge.

All of the above references are incorporated herein in full. The gate structure embodiment of FIG. 5, having silicon oxide-oxide insulator nanolaminates-silicon oxide, are herein used in place of the gate structure provided in those references to support the various embodiments of the present invention. That is, the present invention incorporates the multitude of device structures described in those references to create a multitude of new embodiments which utilize electron trapping in the insulator nanolaminate gate structure shown in FIG. 5, rather than employing floating gates, as recited in many of the above references.

Sample Operation

FIGS. 7A-B and 8 are embodiments useful in illustrating the use of charge storage in the oxide insulator nanolaminate layers to modulate the conductivity of the transistor cell according to the teachings of the present invention. That is, FIGS. 7A-7B illustrates the operation of an embodiment for a novel transistor cell 701 formed according to the teachings of the present invention. And, FIG. 8 illustrates the operation of a conventional DRAM cell 701. As shown in FIG. 7A, the embodiment consists of a gate insulator stack having insulator layers, 710, 708 and 718, e.g. SiO₂/oxide insulator nanolaminate layers/SiO₂. In the embodiment of FIG. 7A, the gate insulator stack having insulator layers, 710, 708 and 718, is made thicker than in a conventional DRAM cell, e.g. 801 and is equal to or greater than 10 nm or 100 Å (10⁻⁶ cm). In the embodiment shown in FIG. 7A a transistor cell is illustrated having dimensions of 0.1 μm (10⁻⁵ cm) by 0.1 μm. The capacitance, Ci, of the structure depends on the dielectric constant, ∈_i, (given here as 0.3×10⁻¹² F/cm), and the thickness of the insulating layers, t, (given here as 10⁻⁶ cm), such that Ci=∈i/t, Farads/cm² or 3×10⁻⁷ F/cm². In one embodiment, a charge of 10¹² electrons/cm² is programmed into the oxide insulator nanolaminate layers of the transistor cell. Here the charge carriers become trapped in potential wells in the oxide insulator nanolaminate layers 708 formed by the different electron affinities of the insulators 710, 708 and 718, as shown in FIG. 7A. This produces a stored charge ΔQ=10¹² electrons/cm²×1.6×10⁻¹⁹ Coulombs. In this embodiment, the resulting change in the threshold voltage (ΔVt) of the transistor cell will be approximately 0.5 Volts (ΔVt=ΔQ/Ci or 1.6×10⁻⁷/3×10⁻⁷=½ Volt). For ΔQ=10¹² electrons/cm³ in the dimensions given above, this embodiment of the present invention involves trapping a charge of approximately 100 electrons in the oxide insulator nanolaminate layers 708 of the transistor cell.

FIG. 7B aids to further illustrate the conduction behavior of the novel transistor cell of the present invention. As one of ordinary skill in the art will understand upon reading this disclosure, if the transistor cell is being driven with a control gate voltage of 1.0 Volt (V) and the nominal threshold voltage without the floating gate charged is ½ V, then if the oxide insulator nanolaminate layers are charged the transistor cell of the present invention will be off and not conduct. That is, by trapping a charge of approximately 100 electrons in the oxide insulator nanolaminate layers of the transistor cell, having dimensions of 0.1 μm (10⁻⁵ cm) by 0.1 μm, will raise the threshold voltage of the transistor cell to 1.0 Volt and a 1.0 Volt control gate potential will not be sufficient to turn the device on, e.g. Vt=1.0 V, I=0.

Conversely, if the nominal threshold voltage without the oxide insulator nanolaminate layers charged is ½ V, then I=μC_ox×(W/L)×((Vgs−Vt)²/2), or 12.5 μA, with μC_ox=μC_i=100 μA/V² and W/L=1. That is, the transistor cell of the present invention, having the dimensions describe above will produce a current I=100 μA/V²×(¼)×(½)=12.5 μA. Thus, in the present invention an un-written, or un-programmed transistor cell can conduct a current of the order 12.5 μA, whereas if the oxide insulator nanolaminate layers are charged then the transistor cell will not conduct. As one of ordinary skill in the art will understand upon reading this disclosure, the sense amplifiers used in DRAM arrays, and as describe above, can easily detect such differences in current on the bit lines.

By way of comparison, in a conventional DRAM with 30 femtoFarad (fF) storage capacitors charged to 50 femtoColumbs (fC), if these are read over 5 nS then the average current on the bit line is only 10 μA. This is illustrated in connection with FIG. 8. As shown in FIG. 8, storing a 50 fC charge on the storage capacitor equates to storing 300,000 electrons.

According to the teachings of the present invention, the transistor cells, having the gate structure with oxide insulator nanolaminate layers, in the array are utilized not just as passive on or off switches as transfer devices in DRAM arrays but rather as active devices providing gain. In the present invention, to program the transistor cell “off,” requires only a stored charge in the oxide insulator nanolaminate layers of about 100 electrons if the area is 0.1 μm by 0.1 μm. And, if the transistor cell is unprogrammed, e.g. no stored charge trapped in the oxide insulator nanolaminate layers, and if the transistor cell is addressed over 10 nS a current of 12.5 μA is provided. The integrated drain current then has a charge of 125 fC or 800,000 electrons. This is in comparison to the charge on a DRAM capacitor of 50 fC which is only about 300,000 electrons. Hence, the use of transistor cells, having the gate structure with oxide insulator nanolaminate layers, in the array as active devices with gain, rather than just switches, provides an amplification of the stored charge, in the oxide insulator nanolaminate layers, from 100 to 800,000 electrons over a read address period of 10 nS.

Sample Device Applications

In FIG. 9 a memory device is illustrated according to the teachings of the present invention. The memory device 940 contains a memory array 942, row and column decoders 944, 948 and a sense amplifier circuit 946. The memory array 942 consists of a plurality of transistor cells 900, having oxide insulator nanolaminate layers in the gate stack, whose word lines 980 and bit lines 960 are commonly arranged into rows and columns, respectively. The bit lines 960 of the memory array 942 are connected to the sense amplifier circuit 946, while its word lines 980 are connected to the row decoder 944. Address and control signals are input on address/control lines 961 into the memory device 940 and connected to the column decoder 948, sense amplifier circuit 946 and row decoder 944 and are used to gain read and write access, among other things, to the memory array 942.

The column decoder 948 is connected to the sense amplifier circuit 946 via control and column select signals on column select lines 962. The sense amplifier circuit 946 receives input data destined for the memory array 942 and outputs data read from the memory array 942 over input/output (I/O) data lines 963. Data is read from the cells of the memory array 942 by activating a word line 980 (via the row decoder 944), which couples all of the memory cells corresponding to that word line to respective bit lines 960, which define the columns of the array. One or more bit lines 960 are also activated. When a particular word line 980 and bit lines 960 are activated, the sense amplifier circuit 946 connected to a bit line column detects and amplifies the conduction sensed through a given transistor cell and transferred to its bit line 960 by measuring the potential difference between the activated bit line 960 and a reference line which may be an inactive bit line. Again, in the read operation the source region of a given cell is couple to a grounded sourceline or array plate (not shown). The operation of Memory device sense amplifiers is described, for example, in U.S. Pat. Nos. 5,627,785; 5,280,205; and 5,042,011, all assigned to Micron Technology Inc., and incorporated by reference herein.

FIG. 10 shows a conventional NOR-NOR logic array 1000 which is programmable at the gate mask level by either fabricating a thin oxide gate transistor, e.g. logic cells 1001-1, 1001-2, . . . , 1001-N and 1003-1, 1003-2, . . . , 1003-N, at the intersection of lines in the array or not fabricating a thin oxide gate transistor, e.g. missing thin oxide transistors, 1002-1, 1002-2, . . . , 1002-N, at such an intersection. As one of ordinary skill in the art will understand upon reading this disclosure, the same technique is conventionally used to form other types of logic arrays not shown. As shown in FIG. 10, a number of depletion mode NMOS transistors, 1016 and 1018 respectively, are used as load devices.

The conventional logic array shown in FIG. 10 includes a first logic plane 1010 which receives a number of input signals at input lines 1012. In this example, no inverters are provided for generating complements of the input signals. However, first logic plane 1010 can include inverters to produce the complementary signals when needed in a specific application.

First logic plane 1010 includes a number of thin oxide gate transistors, e.g. transistors 1001-1, 1001-2, . . . , 1001-N. The thin oxide gate transistors, 1001-1, 1001-2, . . . , 1001-N, are located at the intersection of input lines 1012, and interconnect lines 1014. In the conventional PLA of FIG. 10, this selective fabrication of thin oxide gate transistor, e.g. transistors 1001-1, 1001-2, . . . , 1001-N, is referred to as programming since the logical function implemented by the programmable logic array is entered into the array by the selective arrangement of the thin oxide gate transistors, or logic cells, 1001-1, 1001-2, . . . , 1001-N, at the intersections of input lines 1012, and interconnect lines 1014 in the array.

In this embodiment, each of the interconnect lines 1014 acts as a NOR gate for the input lines 1012 that are connected to the interconnect lines 1014 through the thin oxide gate transistors, 1001-1, 1001-2, . . . , 1001-N, of the array. For example, interconnection line 1014A acts as a NOR gate for the signals on input lines 1012A and 1012B. That is, interconnect line 1014A is maintained at a high potential unless one or more of the thin oxide gate transistors, 1001-1, 10041-2, . . . , 1001-N, that are coupled to interconnect line 1014A are turned on by a high logic level signal on one of the input lines 1012. When a control gate address is activated, through input lines 1012, each thin oxide gate transistor, e.g. transistors 1001-1, 1001-2, . . . , 1001-N, conducts which performs the NOR positive logic circuit function, an inversion of the OR circuit function results from inversion of data onto the interconnect lines 1014 through the thin oxide gate transistors, 1001-1, 1001-2, . . . , 1001-N, of the array.

As shown in FIG. 10, a second logic plane 1024 is provided which includes a number of thin oxide gate transistor, e.g. transistors 1003-1, 1003-2, . . . , 1003-N. The thin oxide gate transistors, 1003-1, 1003-2, . . . , 1003-N, are located at the intersection of interconnect lines 1014, and output lines 1020. Here again, the logical function of the second logic plane 1024 is implemented by the selective arrangement of the thin oxide gate transistors, 1003-1, 1003-2, . . . , 1003-N, at the intersections of interconnect lines 1014, and output lines 1020 in the second logic plane 1024. The second logic plane 1024 is also configured such that the output lines 1020 comprise a logical NOR function of the signals from the interconnection lines 1014 that are coupled to particular output lines 1020 through the thin oxide gate transistors, 1003-1, 1003-2, . . . , 1003-N, of the second logic plane 1024. Thus, in FIG. 10, the incoming signals on each line are used to drive the gates of transistors in the NOR logic array as the same is known by one of ordinary skill in the art and will be understood by reading this disclosure.

FIG. 11 illustrates an embodiment of a novel in-service programmable logic array (PLA) formed with logic cells having a gate structure with oxide insulator nanolaminate layers, according to the teachings of the present invention. In FIG. 11, PLA 1100 implements an illustrative logical function using a two level logic approach. Specifically, PLA 1100 includes first and second logic planes 1110 and 1122. In this example, the logic function is implemented using NOR-NOR logic. As shown in FIG. 11, first and second logic planes 1110 and 1122 each include an array of, logic cells, having a gate structure with oxide insulator nanolaminate layers, which serve as driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N respectively, formed according to the teachings of the present invention. The driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N, have their first source/drain regions coupled to source lines or a conductive source plane. These driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N are configured to implement the logical function of FPLA 1100. The driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N are shown as n-channel transistors. However, the invention is not so limited. Also, as shown in FIG. 11, a number of p-channel metal oxide semiconductor (PMOS) transistors are provided as load device transistors, 1116 and 1124 respectively, having their source regions coupled to a voltage potential (VDD). These load device transistors, 1116 and 1124 respectively, operate in complement to the driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N to form load inverters.

It is noted that the configuration of FIG. 11 is provided by way of example and not by way of limitation. Specifically, the teachings of the present application are not limited to programmable logic arrays in the NOR-NOR approach. Further, the teachings of the present application are not limited to the specific logical function shown in FIG. 11. Other logical functions can be implemented in a programmable logic array, with the driver transistors, having a gate structure with oxide insulator nanolaminate layers, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N and load device transistors, 1116 and 1124 respectively, of the present invention, using any one of the various two level logic approaches.

First logic plane 1110 receives a number of input signals at input lines 1112. In this example, no inverters are provided for generating complements of the input signals. However, first logic plane 1110 can include inverters to produce the complementary signals when needed in a specific application.

First logic plane 1110 includes a number of driver transistors, having a gate structure with oxide insulator nanolaminate layers, 1101-1, 1101-2, . . . , 1101-N, that form an array. The driver transistors, 1101-1, 1101-2, . . . , 1101-N, are located at the intersection of input lines 1112, and interconnect lines 1114. Not all of the driver transistors, 1101-1, 1101-2, . . . , 1101-N, are operatively conductive in the first logic plane. Rather, the driver transistors, 1101-1, 1101-2, . . . , 1101-N, are selectively programmed, as has been described herein, to respond to the input lines 1112 and change the potential of the interconnect lines 1114 so as to implement a desired logic function. This selective interconnection is referred to as programming since the logical function implemented by the programmable logic array is entered into the array by the driver transistors, 1101-1, 1101-2, . . . , 1101-N, that are used at the intersections of input lines 1112, and interconnect lines 1114 in the array.

In this embodiment, each of the interconnect lines 1114 acts as a NOR gate for the input lines 1112 that are connected to the interconnect lines 1114 through the driver transistors, 1101-1, 1101-2, . . . , 1101-N, of the array 1100. For example, interconnection line 1114A acts as a NOR gate for the signals on input lines 1112A, 1112B and 1112C. Programmability of the driver transistors, 1101-1, 1101-2, . . . , 1101-N is achieved by trapping charge carriers in potential wells in the oxide insulator nanolaminate layers of the gate stack, as described herein. When the oxide insulator nanolaminate layers are charged, that driver transistor, 1101-1, 1101-2, . . . , 1101-N will remain in an off state until it is reprogrammed. Applying and removing a charge to the oxide insulator nanolaminate layers, is performed by tunneling charge into the oxide insulator nanolaminate layers of the driver transistors, 1101-1, 1101-2, . . . , 1101-N. A driver transistors, 1101-1, 1101-2, . . . , 1101-N programmed in an off state remains in that state until the charge is removed from the oxide insulator nanolaminate layers.

Driver transistors, 1101-1, 1101-2, . . . , 1101-N not having their corresponding gate structure with oxide insulator nanolaminate layers charged operate in either an on state or an off state, wherein input signals received by the input lines 1112A, 1112B and 1112C determine the applicable state. If any of the input lines 1112A, 1112B and 1112C are turned on by input signals received by the input lines 1112A, 1112B and 1112C, then a ground is provided to load device transistors 1116. The load device transistors 1116 are attached to the interconnect lines 1114. The load device transistors 1116 provide a low voltage level when any one of the driver transistors, 1101-1, 1101-2, . . . , 1101-N connected to the corresponding interconnect line 1114 is activated. This performs the NOR logic circuit function, an inversion of the OR circuit function results from inversion of data onto the interconnect lines 1114 through the driver transistors, 1101-1, 1101-2, . . . , 1101-N of the array 1100. When the driver transistors, 1101-1, 1101-2, . . . , 1101-N are in an off state, an open is provided to the drain of the load device transistors 1116. The VDD voltage level is applied to corresponding input lines, e.g. the interconnect lines 1114 for second logic plane 1122 when a load device transistors 1116 is turned on by a clock signal received at the gate of the load device transistors 1116. Each of the driver transistors, 1101-1, 1101-2, . . . , 1101-N described herein are formed according to the teachings of the present, having a gate structure with oxide insulator nanolaminate layers.

In a similar manner, second logic plane 1122 comprises a second array of driver transistors, 1102-1, 1102-2, . . . , 1102-N that are selectively programmed to provide the second level of the two level logic needed to implement a specific logical function. In this embodiment, the array of driver transistors, 1102-1, 1102-2, . . . , 1102-N is also configured such that the output lines 1120 comprise a logical NOR function of the signals from the interconnection lines 1114 that are coupled to particular output lines 1120 through the driver transistors, 1102-1, 1102-2, . . . , 1102-N of the second logic plane 1122.

Programmability of the driver transistors, 1102-1, 1102-2, . . . , 1102-N is achieved by trapping charge carriers in potential wells in the oxide insulator nanolaminate layers of the gate stack, as described herein. When the oxide insulator nanolaminate layers are charged, that driver transistor, 1102-1, 1102-2, . . . , 1102-N will remain in an off state until it is reprogrammed. Applying and removing a charge to the oxide insulator nanolaminate layers are performed by tunneling charge into the oxide insulator nanolaminate layers of the driver transistors, 1101-1, 1101-2, . . . , 1101-N. A driver transistor, e.g. 1102-1, 1102-2, . . . , 1102-N, programmed in an off state remains in that state until the charge is removed from the oxide insulator nanolaminate layers.

Driver transistors, 1102-1, 1102-2, . . . , 1102-N not having their corresponding gate structure with oxide insulator nanolaminate layers charged operate in either an on state or an off state, wherein signals received by the interconnect lines 1114 determine the applicable state. If any of the interconnect lines 1114 are turned on, then a ground is provided to load device transistors 1124 by applying a ground potential to the source line or conductive source plane coupled to the transistors first source/drain region as described herein. The load device transistors 1124 are attached to the output lines 1120. The load device transistors 1124 provide a low voltage level when any one of the driver transistors, 1102-1, 1102-2, . . . , 1102-N connected to the corresponding output line is activated. This performs the NOR logic circuit function, an inversion of the OR circuit function results from inversion of data onto the output lines 1120 through the driver transistors, 1102-1, 1102-2, . . . , 1102-N of the array 1100. When the driver transistors, 1102-1, 1102-2, . . . , 1102-N are in an off state, an open is provided to the drain of the load device transistors 1124. The VDD voltage level is applied to corresponding output lines 1120 for. second logic plane 1122 when a load device transistor 1124 is turned on by a clock signal received at the gate of the load device transistors 1124. In this manner a NOR-NOR electrically programmable logic array is most easily implemented utilizing the normal PLA array structure. Each of the driver transistors, 1102-1, 1102-2, . . . , 1102-N described herein are formed according to the teachings of the present, having a gate structure with oxide insulator nanolaminate layers.

Thus FIG. 11 shows an embodiment for the application of the novel transistor cells, having a gate structure with oxide insulator nanolaminate layers, in a logic array. If a driver transistors, 1101-1, 1101-2, . . . , 1101-N, and 1102-1, 1102-2, . . . , 1102-N, is programmed with a negative charge trapped in potential wells, formed with the oxide insulator nanolaminate layers, it is effectively removed from the array. In this manner the array logic functions can be programmed even when the circuit is in the final circuit or in the field and being used in a system.

The absence or presence of charge trapped in potential wells, formed by the oxide insulator nanolaminate layers, is read by addressing the input lines 1112 or control gate lines and y-column/sourcelines to form a coincidence in address at a particular logic cell. The control gate line would for instance be driven positive at some voltage of 1.0 Volts and the y-column/sourceline grounded, if the oxide insulator nanolaminate layers are not charged with electrons then the transistor would turn on tending to hold the interconnect line on that particular row down indicating the presence of a stored “one” in the cell. If this particular transistor cell has charge trapped in potential wells, formed by the oxide insulator nanolaminate layers, the transistor will not turn on and the presence of a stored “zero” is indicated in the cell. In this manner, data stored on a particular transistor cell can be read.

Programming can be achieved by hot electron injection. In this case, the interconnect lines, coupled to the second source/drain region for the transistor cells in the first logic plane, are driven with a higher drain voltage like 2 Volts for 0.1 micron technology and the control gate line is addressed by some nominal voltage in the range of twice this value. Erasure is accomplished by driving the control gate line with a large positive voltage and the sourceline and/or backgate or substrate/well address line of the transistor with a negative bias so the total voltage difference is in the order of 3 Volts causing electrons to tunnel out of the oxide insulator nanolaminate layers of the driver transistors. Writing can be performed, as also described above, by normal channel hot electron injection

One of ordinary skill in the art will appreciate upon reading this disclosure that a number of different configurations for the spatial relationship, or orientation of the input lines 1112, interconnect lines 1114, and output lines 1120 are possible.

FIG. 12 is a block diagram of an electrical system, or processor-based system, 1200 utilizing transistor cells with a gate structure having oxide insulator nanolaminate layers. By way of example and not by way of limitation, memory 1212 is constructed in accordance with the present invention to have transistor cells with a gate structure having oxide insulator nanolaminate layers. The same applies to transistors in the CPU, etc., the invention is not so limited. The processor-based system 1200 may be a computer system, a process control system or any other system employing a processor and associated memory. The system 1200 includes a central processing unit (CPU) 1202, e.g., a microprocessor, that communicates with the NOR flash memory 1212 and an I/O device 1208 over a bus 1220. It must be noted that the bus 1220 may be a series of buses and bridges commonly used in a processor-based system, but for convenience purposes only, the bus 1220 has been illustrated as a single bus. A second I/O device 1210 is illustrated, but is not necessary to practice the invention. The processor-based system 1200 can also includes read-only memory (ROM) 1214 and may include peripheral devices such as a floppy disk drive 1204 and a compact disk (CD) ROM drive 1206 that also communicates with the CPU 1202 over the bus 1220 as is well known in the art.

It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device 1200 has been simplified to help focus on the invention. In one embodiment, at least one of the transistor cells, having a gate structure with oxide insulator nanolaminate layers in memory 1212 includes a programmed transistor cell according to the teachings of the present invention.

It will be understood that the embodiment shown in FIG. 12 illustrates an embodiment for electronic system circuitry in which the novel transistor cells of the present invention are used. The illustration of system 1200, as shown in FIG. 12, is intended to provide a general understanding of one application for the structure and circuitry of the present invention, and is not intended to serve as a complete description of all the elements and features of an electronic system using the novel transistor cell structures. Further, the invention is equally applicable to any size and type of memory device 1200 using the novel transistor cells of the present invention and is not intended to be limited to that described above. As one of ordinary skill in the art will understand, such an electronic system can be fabricated in single-package processing units, or even on a single semiconductor chip, in order to reduce the communication time between the processor and the memory device.

Applications containing the novel transistor cell of the present invention as described in this disclosure include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.

CONCLUSION

This disclosure describes the use of oxide insulator nanolaminate layers with charge trapping in potential wells formed by the different electron affinities of the insulator layers. Two different types of materials are used for the nanolaminated insulator layers. The two different types of materials are transition metal oxides and silicon oxycarbide. In the case of transition metal oxide layers, these are formed by ALD and have atomic dimensions, or nanolaminates, with precisely controlled interfaces and layer thickness. In the case of silicon oxycarbide, these are deposited using chemical vapor deposition techniques since an ALD process has not yet been developed. The gate insulator structure embodiments of the present invention, having silicon oxide-metal oxide-silicon oxide insulator nanolaminates, are employed in a wide variety of different device applications.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.